1. Field of the Invention
The present invention relates to an image recording apparatus for recording an image with recording elements, and more particularly to such apparatus utilizing light-emitting elements such as light-emitting diodes.
2. Description of the Prior Art
In the field of facsimile receiver or the computer output peripheral, there is already known a printer for image recording with an array of recording elements such as light-emitting diodes (LED). The printing head of such printer utilizing LED's as the recording elements is generally constructed as shown in FIG. 1, in which the printing head 1 comprises a linear array of for example 4096 LED's, which are divided into 64 blocks each containing 64 elements. Cathodes of 64 LED's in each block are connected to a common line B1, B2, . . . , or B64 which is common to each block, while anodes of the LED's of a same ordinal number in different blocks are connected to a common line D1, D2, . . . , or D64. Such LED printing head is generally driven by so-called dynamic driving method, in which the common lines D1-D64 for the anodes are selectively given a determined voltage V through unrepresented switching elements controlled by parallel pixel signals, while the different blocks of the cathodes are grounded in succession in synchronization with the supply of said pixel signals.
In the LED printer utilizing such dynamic drive, subsidiary scanning is achieved by a movement of a photosensitive member such as a photosensitive drum or sheet at a constant speed at the end of a main scanning process achieved by the above-explained dynamic drive. FIG. 2 shows an example of arrangement for achieving such function, wherein shown are an LED printing head 1, a linear array 2 of plural LED's mounted in said printing head 1, a photosensitive drum 3 rotated at a constant speed in a direction indicated by an arrow, and a rod lens array 4 for focusing the light from the LED array 2 onto the photosensitive drum 3. In such arrangement, however, since the photosensitive drum 3 is rotated at a constant speed even during the above-described dynamic scanning process, there may result a stepwise aberration in the exposure positions between different blocks due to the presence of an inactive period of LED between different blocks. Also in such printer the LED's have to be driven with a high efficiency since the light of an LED is much weaker than the light of a laser or the like and also since only 3 to 5% of the entire emitted light can be focused on the photosensitive member 3.
In such conventional LED printer, the image is formed on the photosensitive member by turning on or off each LED according to binary digital image signals. Consequently, a halftone image such as a photograph can only be reproduced by so-called dither process, which not only complicates the circuitry but also leads to a lowered resolving power as the image density is represented in the dither process by the number of plural dots constituting a pixel area.